Rudiocyon amplidens, Hunt, 2011

Hunt, Robert M., 2011, Evolution Of Large Carnivores During The Mid-Cenozoic Of North America: The Temnocyonine Radiation (Mammalia, Amphicyonidae), Bulletin of the American Museum of Natural History 2011 (358), pp. 1-153 : 54-57

Rudiocyon amplidens , new species Figures 22 , 23

Temnocyon large species: Stock, 1933b: 36.

TYPE: LACM 480, partial left mandible with p4–m2, right m2, right p4 protocone, and a partial braincase, from the John Day Formation, CIT loc. 31, 1.5 to 2 mi south of the Johnson Ranch [now Tom Campbell Ranch] at the mouth of Rudio Creek, from gray beds exposed on the east side of Rudio Creek, south of the North Fork of the John Day River, Grant County, Oregon, collected by E.L. Furlong (,1930).

DISTRIBUTION: Early or mid-Arikareean, John Day Formation, Grant County, Oregon.

ETYMOLOGY: From the Latin, amplus, ‘‘large,’’ and dens, ‘‘tooth,’’ in reference to the robust teeth in this species, the largest of the John Day temnocyonine beardogs.

DIAGNOSIS: Largest John Day temnocyonine species, m1 length, 28.7 mm, m1 without metaconid thereby distinguished from Temnocyon ; m1 hypoconid centrally placed on talonid, no entoconid; m1 labial cingulum sinuous, not straight as in Delotrochanter ; p4 narrower than in Delotrochanter and Mammacyon ferocior but similar in this respect to M. obtusidens , T. ferox , and T. macrogenys . Relatively short m2 (E/F ratio,,1.9, the highest value for any temnocyonine), not elongate as in Mammacyon , with blunt protoconid and hypoconid the principal cusps, aligned one behind the other along the midline of m2; small m2 paraconid occupies anterointernal corner; no metaconid or entoconid. Mandible deep below m1–2 (43 mm), more so than in any other John Day temnocyonine.

REFERRED SPECIMENS: None.

DESCRIPTION: The partial mandible (length,, 114 mm) retains p4–m2. The dorsal border of the jaw is damaged anterior to p4 but has alveoli for p3. Depth of mandible:, 36 mm below p3; 37 mm below the posterior root of p4; 40 mm below the m1 protoconid; and 43 mm below the anterior root of m2; the anterior masseteric fossa is present below the m2 talonid.

The p4 is narrow and similar in form to the p4 of Mammacyon obtusidens and Eyerman’s type of T. ferox . The tooth is tall, the principal cusp 5 mm higher than the m1 paraconid. A fine enamel ridge on the anterior slope intersects the cingulum. A large labially situated posterior accessory cusp occurs above a small basal cusp. The anterior (8.1 mm) and posterior (10.0 mm) widths of p4 demonstrate a narrow p 4 in Rudiocyon , which compares with these p4 widths of M. obtusidens (7.4, 9.7 mm).

The m1 measures 28.7 mm in length, 13.2 mm in width at the base of the protoconid, 11.8 mm in talonid width, and lacks both a metaconid and entoconid. This is a massive carnassial, the protoconid much taller than the flanking paraconid and hypoconid. The hypoconid is as large as the paraconid and nearly equal in height, indicating the important role played by this pestlelike cusp in crushing. It occupies the center of the talonid, rather than the more labial placement seen in T. macrogenys . Anterior to the blunt hypoconid is a small vertical swelling of enamel on the posterior face of the protoconid. It measures 2.4 mm in width, 3.7 mm in height. Above and lingual to this swelling is a thin enamel ridge also situated on the posterior face of the protoconid. This is not a vestige of the metaconid but rather a remnant of a thin protoconid crest that in a plesiomophic m1 forms an incisure with the metaconid. It remains as a low narrow ridge in temnocyonines that have recently lost the metaconid, and is absent in species that have long been without this cusp. A sinuous (not straight as in Delotrochanter ) cingulum surrounds the base of the tooth on the labial side and on the posterior half of the lingual side, but is indistinct on the lingual side of the trigonid. The lingual cingulum medial to the hypoconid contains a small notch.

The m2 is not elongate and narrows posteriorly, differing from Mammacyon : greatest trigonid width is 9.1 mm, talonid width 8.4 mm. The protoconid and smaller hypoconid, both blunt crushing cusps, are anteroposteriorly aligned with the m1 hypoconid. There is variation in the cusp placement of right and left m2: the left m2 shows these two cusps centrally placed, whereas the right m2 shows them closer to the labial margin—it is possible that the latter tooth represents another individual. There is no m2 metaconid or entoconid. However, as in other large temnocyonine species, the anterolabial cingulum of the m2 trigonid is protuberant.

The right P4 protocone, the only remnant of the upper dentition, is similar in size and blunt conical form to that cusp in large species of Mammacyon . It measures 10.9 mm in anteroposterior width and compares closely with the protocone of ACM 34–41, the holotype of M. obtusidens . The protocone was encircled by a thickened cingulum.

DISCUSSION: Rudiocyon amplidens is a carnivore about the same size as Mammacyon ferocior or Delotrochanter oryktes , and is somewhat smaller than T. macrogenys . Thus, the species was one of the larger North American temnocyonines. The m1/m2 length ratio of R. amplidens (,1.9) distinguishes it from M. ferocior (1.6), T. macrogenys (,1.6), and the species of Delotrochanter (,1.6–1.7). In addition, its p4 is not as broad as the M. ferocior p4, nor does it have an m1 metaconid as in T. macrogenys . Delotrochanter oryktes and D. major also differ from R. amplidens in having a short, posteriorly wide p 4 in contrast to the more narrow p4 of Rudiocyon .

In its p4–m1, in the form of the mandible, but particularly the P4 protocone, R. amplidens shows an evident similarity to Mammacyon . However, M. obtusidens has a longer m2 and a much smaller m1 than R. amplidens , and M. ferocior has a boader p4 and a much more elongate m2.

Mammacyon mandibular dentitions are distinguished by the elongate m2 and are known only from the Arikareean of the Great Plains. Because the only evidence of Mammacyon in the John Day basin is a maxilla (LACM 5386) from Haystack Valley, the nature of m 2 in that species cannot be determined. T. altigenis is the only temnocyonine where m2 variation can be estimated and the fossils are not from a single stratigraphic horizon. The variation in m2 length in the T. altigenis sample indicates that the hypodigm has a range of 1.67–1.83 for the m1/m2 length ratio (table 6). Yet the difference between the m1/m2 length ratio of R. amplidens (1.89) and those of M. obtusidens – M. ferocior (1.57–1.6) exceeds that range. LACM 480 cannot be included in Mammacyon unless the genus is more broadly defined to include more pronounced variation in dental proportions, particularly with regard to m2.

A partial braincase accompanies the lower jaw and presumably was collected with it. It is remarkable how similar most temnocyonine skulls are in braincase width, a proxy for volume of the cranial cavity (table 7): five of the larger species all measure from 62 to 67 mm, R. amplidens among them. The braincase of LACM 480 cannot be compared with temnocyonines with inflated frontal paranasal sinuses because the frontal region anterior to the braincase was not preserved.

The p4–m2 of Rudiocyon shows similarities to these same teeth in T. ferox . These include the squared posterior heel and cusp pattern of p4; the form of m1, which suggests that the reduced metaconid in T. ferox could have finally been lost in LACM 480; and the occlusal form of the relatively short m 2 in both species. LACM 480 came from gray tuffaceous sandstone of the Kimberly Member along Rudio Creek a short distance above a supposed correlative ash of the Deep Creek tuff, dated elsewhere at,27.9 Ma. If this age for the Rudio Creek ash is correct, it would appear to be a much older carnivore than T. ferox , which is a smaller and possibly much younger species, and in this case an ancestordescendant relationship would be unlikely. However, if the correlative ash along Rudio Creek, identified by Fisher ( Fisher and Rensberger, 1972: 14) as the Deep Creek tuff in fact represents a much younger undated ash-fall event, then the relative ages of R. amplidens and T. ferox are unspecified.

Figures

Fig. 22. Holotype mandible of Rudiocyon amplidens (LACM 480), with left p4–m2, probably from 1.5 mi south of the Johnson Ranch, from gray beds exposed on the east side of Rudio Creek, Kimberly Member, John Day Formation, Grant Co., Oregon. A, labial and B, lingual views. This is the largest John Day temnocyonine.

Fig. 23. Occlusal stereopair of the holotype dentition (p4–m2) of Rudiocyon amplidens (LACM 480). Note absence of m1 and m2 metaconids and narrow compressed p4.